1. Field of the Invention
The present invention relates to a defect inspecting apparatus and a defect inspecting method for inspecting a defect on the surface of an object to be inspected, such as a semiconductor wafer, a liquid-crystal glass substrate, and the like.
2. Description of Related Art
In general, the process of manufacturing semiconductor wafers (hereafter simply referred to as wafers), liquid-crystal glass substrates, and the like includes the so-called macroscopic inspection for macroscopically inspecting defects, such as flaws and dust on wafer surfaces, faulty application of a resist, and faulty exposure. For example, as the macroscopic inspection of the wafers, the visual inspection is conducted by an operator mainly according to the following two methods.
In one method, intensive illumination light is applied to the wafer surface, and dark field observation is effected while tilting or rotating the wafer at various angles. If there is a defect, the defect is observed as being a difference in brightness or color when the inclination or the angle meets a certain condition.
In a second method, diffusing illumination, in particular, regularly reflected light is used to effect bright field observation. If there is a defect, the defect is observed as being a slight difference in brightness or color.
This visual macroscopic inspection, however, requires the operator to be near the wafer, and therefore may cause contamination of the wafer. That is, it is difficult to maintain the degree of cleanliness and the improved quality of the wafer. Further, this visual macroscopic inspection depends on the skill of the operators, and the same operator fails to detect the same defect as the case maybe. That is, there is a drawback in ensuring stable quality, and large expense and time are required in educating personnel and keeping the skilled operator. Accordingly, there has been a demand toward automated macroscopic inspection.
As the automated macroscopic inspection, such a method is conceivable that an object to be inspected, such as a wafer, is imaged by an imaging device such as a CCD, and image data thereof is subjected to image processing so as to digitally extract a defect. However, the imaging instead of the visual observation by the operator encounters the following problems.
Since the inspection under dark field observation is effected using diffracted light, an angle of observation at which a defect can be confirmed varies depending on type of the object to the inspected, so that a mechanism for adjusting the observation angle is required.
The inspection under the bright field observation utilizes regularly reflected light, but depending on the size of the region of a diffuse illumination plate, the inspection is adversely affected by the phenomenon of a magic mirror due to the warp of the object to be inspected and by the action of diffraction. Hence, the imaging conditions change, so that a phantom defect occurs even if the object to be inspected has no defect.
If a wafer having repeated patterns is normally imaged by the imaging device, moire occurs due to the interaction between the pitch of the pixels of the imaging device and the pitch of the patterns, and constitutes a factor of false defects. Although it is conceivable to adopt a countermeasure for suppressing the generation of moire by using a zoom lens, it is impossible to completely eliminate the moire since the vertical and horizontal pitches of the chip patterns and the vertical and horizontal pitches of the pixels of the CCD generally differ from each other.
Furthermore, as a method of detecting a defect from repeated patterns, such as chips formed on the wafer, the pattern matching method is known in which the presence or absence of a defect is detected on the basis of comparison between two adjacent patterns.
However, if the two patterns are merely compared, the detected defect, which is on one pattern, is recognized as defect information on both the two compared patterns, and it is impossible to specify which one of the patterns has the defect. If there is only one defect in a repeated pattern, the defect can be specified simply by comparing the pattern with a further adjacent pattern. However, if defects are present continuously or if the number of defects becomes numerous, it is difficult to specify the pattern or patterns where the defects are present. Particularly in cases where the defects geometrically overlap with each other, the determination as to which defects belong to which patterns becomes complicated.